Versatile transformations of biphenylenes by transition-metal catalysts and application for the synthesis of polycyclic hydrocarbons

Transition-metal-catalyzed C - C bond activation is one of the most fascinating reactions to construct new C - C bonds because it can convert a simple carbon skeleton directly to a highly complicated carbon framework. In particular, transformations of biphenylene via C - C bond activation are well-known for the synthesis of polycyclic hydrocarbons such as phenanthrene and tetraphenylene. Since the products derived from biphenylene were still limited, there was room for investigation of biphenylene chemistry. Herein we summarize our recent progress for the new reactions of biphenylene by transition-metal catalysts to synthesize various polycyclic hydro¬ carbons. We achieved an iridium-catalyzed intermolecular [4+1] cycloaddition of biphenylenes with alkenes to give 9,9-disubstituted fluorene derivatives and we conducted a mechanism study by DFT calculations. In addition, we developed a new strategy to cleave the sterically hindered C - C bond of biphenylene by using an alkene and an alkyne moiety, respectively, which were used as both a directing group and a reaction site. As a result, dihydrobenzofluoranthene and axially chiral benzofluoranthene derivatives were obtained, which have unique photophysical properties. Furthermore, we achieved skeletal rearrangement via the cleavage of two different C- C bonds by a gold catalyst at ambient temperature and we elucidated reactivity and regioselectivity by experi¬ mental and computational studies.